Hi, just a brief introduction: I used to play poker for a living, played up to NL100 on Fulltilt.com and on Italian sites after the online poker regulation in Italy. I wasn't a big winner, but did quite well at least up to NL50.
Eventually I stopped playing about five years ago, due to burnout. I'm happy with my decision, life's going well and I don't regret it.
Recently the love for the game kicked in again and I deposited some money and starter playing NL5 zoom. I have no plans to play for a living anymore, still I want to be a winner and to study the game as much as I can while enjoying it.

Now the bitter part: I played under 20k hands, so nothing really relevant, but I feel like I'm not supposed to beat this level at the moment, which is kinda a surprise to me.
I understand that poker has gone a lot far (solvers weren't a thing 5 years ago), but what bothers me the most is the fact that some concepts that were widely considered true five years ago seems wrong now, at least in my small Experience so far.

I don't really know if that's due to the fact I'm playing quite lower than what I was used to or the fact that people is a lot better.
I'll try to point out some general impressions, so you can give me your opinion.

• I feel like the necessary hand strenght to win at showdown is much higher. Overpairs are not winning big hands like it was before, top sets are constantly losing to flushes, flushes to nut flushes etc. While when I've the nuts rarely I'm able to stack someone. This could be just variance, could be me playing bad or could be that NL5 just play a lot passive than what I'm used to (preflop dynamics doesn't seems so passive tho).

• ranges with I'm willing to stack off pre flop seems too wide given what capable villains are showing off. Rarely I see someone broke pre with AK or JJ-QQ and almost never with weaker hands. Even in BvsB situation.
  Again could be just a micros' thing.

• 5 years ago calling a 4bet pre was considered an huge mistake. Today I feel like I should have a range to call 4 bets with, the value part of this range could be JJ-QQ, AK while in position given what said in the point before. Not really sure what I should add to balance it tho

• bet sizing is much different today. When I was playing a 2/3 pot c-bet was the to go size. Now I see much smaller sizings. I can see the merits of this, but I should study the math behind this (if you could suggest a video about this topic would be nice)

I don't want to put out too much things, so I'll stop there. Thanks to everyone that will help me by answering